In an optical disk drive apparatus, it is necessary to control a focus servo to ensure that the surface of the disk remains always within the focal depth of the objective lens since the axial deflection of a disk can range from about 100 to 200 micrometers while the focal depth of an objective lens is only from about 1 to 2 micrometers. FIG. 6 shows a conventional configuration for such a focus servo. A focus error signal to be used for focus servo control is detected by using a known method such as an astigmatic method, a knife edge method, a beam size method, or a wedge prism method. A focus error signal to be detected by using any of the above methods has a so-called S-curve characteristic in the vicinity of an in-focus position of an objective lens as shown in FIG. 7.
In the S-curve of FIG. 7, an ordinate and an abscissa indicate a focus error signal and a relative distance between a disk 600 and an objective lens 604, respectively. In FIG. 7, a direction from the left to the right is one in which the objective lens 604 approaches the disk 600, and the zero-crossing point of the S-curve represents a position of the objective lens 604 at the time when the light beam is focused.
The operations of conventional focus servo control are described by reference to FIG. 7 and FIG. 8. The abscissa in FIG. 8 indicates time. If focus acquisition begins outside the range AB in FIG. 7, focus acquisition fails since the focus servo loop offers positive feedback. Focus acquisition, therefore, needs to be started within the range AB shown in the figure. Accordingly, before focus acquisition is started, a contact of a switch 618 is switched to a ramp circuit 616 to keep the focus servo loop open. After the objective lens 604 is moved away from the disk 600 by means of the ramp circuit 616, it is gradually brought close to the disk 600 again. And, when a zero-crossing detecting circuit 612 detects a zero-crossing of a focus error signal whose peak has passed, a microprocessor 614 switches over the contact of the switch 618 to a focus error signal generator 610 to close the focus servo loop. Thus, a focus actuator 606 of an optical head 608 is provided with a drive signal by means of a phase compensating circuit 620 and a focus actuator driver 622 to control the position of the objective lens 604 so as to ensure that the surface of the disk remains within the focal depth of the objective lens 604. The functions of the phase compensating circuit 620 and the focus actuator driver 622 are well-known to those skilled in the art and further description is omitted.
The ramp circuit 616 generally generates a sawtooth wave of a relatively low frequency. However, if a travelling speed of the objective lens 604 is too high at the time of zero-crossing of a focus error signal, the objective lens 604 will overshoot, as indicated by the broken line 800 in FIG. 8, and focus acquisition will fail. Such a change of a travelling speed of the objective lens can occur due to variations in focus error signals between optical systems, catching of the focus actuator, abrupt deflection of the disk surface and the like. In addition, if the optical disk drive apparatus itself is installed on the tilt, the travelling speed of the objective lens 604 widely differs depending on how gravity affects the objective lens 604.
To solve the above problem, one proposed method decreases the generated voltage of the ramp circuit 616 to slowly move the objective lens 604. This method, however, requires excessive time for focus acquisition. To solve the problem of a change of travelling speed of the objective lens 604 due to gravity, another method provides means for detecting an installation angle of the optical disk drive apparatus and changing a generated voltage of the ramp circuit 616 based on the detected angle; this method requires the extra means for detecting the installation angle.